Latex recycling process



March 3, 1964 L. E. LEFEVRE 3,123,579

LATEX RECYCLING PROCESS Filed May 5, 1960 La/ex 801 /17 1 F 1 6/ fi y T n/r 15 Reservoir /1 INVENTOR. Maya E. Ze/w'e ZJAAAZJ- HGENT' United States Patent Delaware Filed May 5, 1%@, Ser. No. 27,200 5 Claims. (Cl. 260M296) This invention relates to a latex recycling process wherein a fresh surface is maintained on a bath of latex of an organic, thermoplastic material. In particular, it relates to such a process wherein the bath surface is continuously renewed while retaining a constant bath level.

Many fabrication techniques employing aqueous polymer latexes use a bath of latex which is exposed to the atmosphere. For example, some articles are made by dipping a form into the bath of latex. Other procedures involve coating a substratum by passage of the substratum in close proximity to the surface of the bath of latex and picking up the desired coating from the meniscus formed between bath and substratum. In this latter instance it is apparent that the level of the bath must be kept constant within the limits of the ultimate meniscus height.

Such latex baths present problems. For example, it is known that a skin may form on some latexes when exposed to the air for prolonged periods. Also, a substratum, either as a belt or roll, when passed over a latex bath, will usually not be as wide as the latex bath so that a fresh surface over the entire bath surface will not necessarily be continuously renewed by the removal of latex from the bath surface onto the substratum. An additional problem with an exposed bath surface is the chance for contamination of the bath from the atmosphere.

The prior known conventional means for maintaining liquid bath levels substantially constant usually involve mechanical regulating and metering devices, such as float controlled valves, nozzles and the like. With polymer latexes these frequently become coated or clogged with coagulum or coalesced latex particles. This coating or clogging impairs the mechanical performance of the regulating device to the point where its usefulness for its intended purpose is questionable. It is to be desired to keep such moving parts in contact with a polymer latex to a minimum.

In view of the aforementioned and other difficulties attendant the provision of a constant bath level while simultaneously providing a continuously fresh surface to a bath of an aqueous polymer latex, it would be deisrable to have a latex bath which overcame these difficulties. The provision of a process which achieves those goals is the principal obiect of this invention.

The above object is accomplished by means of the process wherein fresh latex is metered into a bath of the same latex at such a rate to provide a continuously renewed surface without creating turbulence in said surface, the excess of that metered into said bath which is not used up through fabrication caused to overflow and be recycled into the fresh latex being introduced into said bath.

The process may be employed with any latex of any polymeric material. The only latex properties of material concern are mechanical stability and fluidity or viscosity. It should be apparent that the latex must be capable of being pumped or at least flowed through pipes and conduits. Because of this permissive latitude of latex type, it is possible to employ a wide variety of polymer types. Thus, there may be used the well-known aqueous latexes of synthetic polymers, exemplary of which are the homopolymers, copolymers, and interpolymers of vinyl and vinylidene halides, such as vinyl chloride and vinylidene chloride; of monoalkenyl aromatic monomers, such as styrene, vinyl toluene, alpha-methyl styrene, and the chicrinated styrenes; of the alkyl, cycloalkyl, and aryl acrylates, such as methyl, ethyL'butyl, Z-ethylhexyl, dodecyl, cyclohexyl, and phenyl acrylates; of the alkyl, cycloalkyl, and aryl alkacrylates, such as methyl methacrylate, ethyl ethacrylate, cyclohexyl methacrylate, and phenyl methacrylate; of the vinyl esters of alkanoic acids, such as vinyl acetate and vinyl propionate; of the vinyl ethers such as 2-chloroethyl vinyl ether; of the unsaturated aldehydes and ketones, such as acrolein and methyl isopropenyl ketone; and of the open chain conjugated dienes, such as butadiene-1,3 and isoprene. Many of the above latexes are commercially available products. For example, styrene/ butadiene, polyvinyl acetate, vinyl chloride/vinylidene chloride, vinylidene chloride/acrylonitrile, butadiene/acrylonitrile, and acrylate-containing latexes are well known and readily available through commercial sources. The process is equally operable with the aqueous latexes of natural polymeric materials, such as Hevea latex. Also operable are those stable latex-like dispersions which result from the suspension of finely divided solids of polymeric materials, such as the polyolefins, including polyethylene, polypropylene, copolymers of ethylene and propylene and polyisobutylene; the condensation polymers, such as the polyamides, including polyhexamethylenediadiparnide, and polyesters, including polyethylene-terephthalate. As with the emulsion polymerizates and the natural latexes, it is only necessary with these latter latex-like dispersions that they have adequate mechanical stability to withstand the shearing forces accompanying the flow that occurs in the process. For ease of description all of these latexes and latex-like dispersions will be referred to as latexes.

The latexes may be of any polymer solids content and of any particle size distribution as long as the requisite minimum stability and fluidity are present. With those latexes which do not exhibit the needed mechanical stability, it is frequently observed that the required stability may be achieved by incorporating in the latex small amounts of additional wetting agent as a post stabilizer.

Latexes that contain the conventional additives which are commonly included in latex formulations can also be employed in the process. Typical of these are heat and light stabilizers, antioxidants, fungicides and other similar agents, dyes, pigments, and fillers. It should be apparent that these additives should not adversely affect the required minimum stability to storage and mechanical stress nor the fluidity of the latex formulation.

The operation of the process and the advantages and benefits derivable therefrom will be more apparent from the following description and the appended drawing which illustrates a preferred embodiment of the invention. In the drawings--- FIGURE 1 represents in schematic outline a procedural sequence for carrying out the process, and

FIGURE 2 represents a particular design of la.ex bath vessel.

In the illustrated embodiment of the process there is provided a reservoir 10 for holding a relatively large supply of latex and for intermixing recycled latex with fresh latex. For this latter purpose there may be known agitation means (not shown) included in the design of the reservoir 10. For maintaining the inventory of latex in the reservoir 10 there is a supply tank 11 connected to the reservoir 10 by a conduit 12. It is frequently desired to maintain the quantity of latex in the reservoir 10 essentially constant to assure that the ratio of fresh latex to recycled latex is as constant as possible. This may be achieved by a number of known ways, such as the imposition on the supply tank 11 of a pressure head using an inert gas, such as nitrogen, by the use of metering pumps, or by similar means.

The intermixed fresh/recycle latex is pumped from the reservoir 10 using a centrifugal pump or other suittion rates.

able metering pumping means 13 through conduit 14 to a filter 15 to screen out coagulum, contamination that may have entered the latex in the bath, and other undesirable materials, then through conduit 16 to the latex bath vessel 17. When the overflow is to be recycled, that efiluent is conducted back to the reservoir through conduit 18. It will be appreciated that many arrangements of the element of apparatus and their design will be possible. It is the sequence of operations and combination of steps which are controlling.

A preferred design for the latex bath vessel is illustrated in FIGURE 2. In this preferred embodiment the vessel consists of a container 20, inlet 2-1 and discharge 22 ports, and a pattern of weirs and battles that cause desirable fiow characteristics. The container 20 preferably has cylindrical or divergent side walls that permit maximum exposed surface for a given volume of latex, that allow for uniform flow patterns without latex being entrapped in corners, and that permit easy cleaning when it is desired to change latexes.

In the bottom and near one end of the container there is located a port 21 to which is aflixed a conduit 23 (corresponding to 16 in FIGURE 1) leading from the reservoir 15) through the metering l3 and filtering 15 means. Through this port 21 latex is introduced into the container 20. Near the opposite end of the container 20 and also in the bottom there is located a second port 22 to which there is atiixed a conduit 24 (corresponding to 18 in FIGURE 1) leading back to the reservoir 16.

Extending upwardly from the bottom of the container 20 is a first weir, 25 preferably, occupying the full cross section of the container 20 up to within a small distance of sides of the container 20. Preferably, the height of the first weir 25 should be approximately the level of latex which will be employed in the bath. This first weir 25 is so located as to place the inlet port 21 between it and the nearest end of the container 20. This position of weir 25 isolates a portion of the container 2t) into a surge chamber A and settling pot for the infiuent latex.

Projecting downwardly from the top of the sides of the container 25 is a baffle 26. This bafile 26 must project below the top surface of the first weir 25 to assure that the infiuent latex will enter the operating portion of the vessel without short circuiting directly across the surface of the bath to the outlet. Preferably the baffie 26 should come as close to the bottom of the container 2% as will permit the desired flow rates to be used without creating turbulence.

Near the outlet end of the container 20* there is positioned a second weir 27 similar in construction and of equal height to the first weir 25. This second weir 27 isolates the turbulence (e.g. in the vortex) created by the overflow from the main body of the container 20 and in conjunction with the first weir 25 provides a bath of constant level.

The present process and apparatus provide the desired benefits independent of the size of apparatus or produc- The dimensions and capacity of equipment to be used depend upon the fabricating technique employed, the production rates desired, preferences of the individual, and upon similar reasons. For practical reasons it is preferred that an excess of bath surface be provided over that amount of surface which 18 used in the fabricating technique used.

In operation, the influent latex enters the bath, travels over the first weir, under the baffle, rises steadily toward the surface, and excess flows in essentially planar flow over the second weir to the outlet port. By essentially planar flow is meant that a layer of the latex of substantially constant depth flows over the top of the second weir. By means of this bath vessel design the surging commonly resulting from metering device is dampened out before the operating surface of the bath is reached, undesirable particles which may pass the filter are allowed to settle in the surge chamber, the level of the bath is kept constant, the surface is continuously renewed, and the possibility of influent latex short circulating across the surface of the bath is prevented.

Mechanical equivalents may be employed for the structures illustrated. Thus, the second weir may be made out of the end plate of the container with the effluent latex spilling into a catch basin. Also, the invention is not limited to the precise disposition of the various elements making up the vessel but only to their relative positions. It is necessary that the operating surface of the latex bath fall between an overflow weir and a bafile plate that causes the introduction of the influent latex to enter the operating portion of the bath in essentially laminar non-turbulent flow at a point near the bottom of the container.

The effluent latex may be discarded or disposed of in any desired manner. However, for reasons of economy it is preferred to recycle the efiiuent latex through the process repetitively. For achieving this purpose a conduit is afiixed to the outlet port and leads back to the reservoir. When equilibrium is reached, there will be a constant ratio of fresh latex to that which has passed through the bath. This is of vital importance to any fabrication technique that depends upon the film formability or other similar property of the latex.

If desired with any given latex, any of the elements in the process may be provided with heating or cooling means to maintain the latex at a temperature at which it is most stable, at which its properties are most favorable for the fabrication technique employed, or for other reason. For example, with the crystalline polymers, such as many of the vinylidene chloride copolymers, the film formaoility of the latex requires inter alia that the polymer particles be of predominantly amorphous state. Crystallinity is dependent in a large part with a particular polymer on a time/ temperature relationship. In such a case it would be desirable to cool the reservoir to extend the useful life of the polymer.

The elements used in the apparatus described supra may be of any suitable material. For example, the usual styrene/butadiene latexes permit the use of almost any structural material, including wood, plastics, and steel, sheet metal and other metals. The vinyl and vinylidene halides, on the other hand, require the use of stainless steels, nickel, and glass or plastic coated metals. These considerations will be knOWn to the skilled worker.

The following exemplary description will illustrate the operation of the process and apparatus with a specific latex. It should be understood that the invention is not limited to the specific latex, to the flow rates used, or to any apparatus other than the latex bath vessel design or logical equivalents.

The process Was employed in a continuous coagulation technique with an aqueous latex of a copolymer prepared from about 97 percent by weight vinylidene chloride and 3 percent by weight of acrylonitrile. In this technique a deposition roll is wetted with an aqueous electrolyte solution and is then rotated so that the wetted surface approaches in close proximity to the surface of the latex bath. A coating of latex is deposited on the roll through the meniscus formed between the roll and the latex bath.

In the process the stainless steel latex bath vessel was slightly narrower than the diameter of the deposition roll and slightly longer than the length of that roll. The vessel was a half cylinder. The arrangement of ports, weirs, and baffle was as shown in FIGURE 2 of the annexed drawing.

Employing this process with the aforementioned latex it was possible to control the latex level in the bath within inch. The surface of the bath remained clean and free of contaminants as evidenced by a freedom of slubs and similar defects in the deposited coating and the film resulting therefrom. The surface of the latex was free of any skinning over of the bath.

The overflow rate of latex could be varied between 0.4

and 1.6 parts by weight of latex per part of dry film prepared without loss of the beneficial results. The overflowed latex was returned to a reservoir where it was intermixed with a large volume of fresh latex and the so-formed blend pumped through a filter to the latex bath.

In contrast, when the latex feed line pumped directly into a bath of similar geometry but without baffles and weirs, the non-uniform bath surface caused serious variations in gauge thickness of the film, the latex outside of that directly in line with the roll was entrapped and became contaminated, the temperature of the latex bath varied causing coagulation difliculties. The process was considered totally unsatisfactory.

What is claimed is:

1. A process for providing a liquid bath of an aqueous latex-like dispersion of an organic, polymeric material so that the surface of said bath is continuously renewed, said process comprising the continuous metering of said aqueous latex-like dispersion from an inventory of said dispersion through a surge and settling chamber then in continuous non-turbulent planar flow into the bottommost portion of a bath of said dispersion at a rate in excess of that at which said dispersion is removed from said bath by the fabricating technique employed, causing the unused excess of said dispersion to be removed from the surface of said bath in essentially planar flow, the

rate at which the introduced latex is fed into said bath being essentially constant and also being equal to the total amount of latex removed therefrom.

2. The process claimed in claim 1 wherein said unused excess of said dispersion removed from said bath is returned to said inventory and fresh latex is supplied to said inventory at a rate to maintain the ratio of fresh latex and said unused excess latex substantially constant.

3. The process claimed in claim 1 wherein said latexlike dispersion introduced into said bath is filtered and is caused to pass through a settling zone prior to introduction into the bottommost portion of said bath.

4. The process claimed in claim 1 wherein said latexlike dispersion is the emulsion polymerizate of at least one olefinically unsaturated monomer.

5. The process claimed in claim 4 wherein said emulsion polymerizate is of an interpolymer composed of vinylidene chloride with any remainder of at least one monoethylenically unsaturated comonomer.

References Cited in the file of this patent UNITED STATES PATENTS 1,958,118 Szegvari May 8, 1934 2,058,044 Spencer Oct. 28, 1936 2,712,161 Moss July 5, 1955 

1. A PROCESS FOR PROVIDING A LIQUID BATH OF AN AQUEOUS LATEX-LIKE DISPERSION OF AN ORGANIC, POLYMERIC MATERIAL SO THAT THE SURFACE OF SAID BATH IS CONTINUOUSLY RENEWED, SAID PROCESS COMPRISING THE CONTINUOUS METERING OF SAID AQUEOUS LATEX-LIKE DISPERSION FROM AN INVENTORY OF SAID DISPERSION THROUGH A SURGE AND SETTLING CHAMBER THEN IN CONTINUOUS NON-TURBULENT PLANAR FLOW INTO THE BOTTOMMOST PORTION OF A BATH OF SAID DISPERSION AT A RATE IN EXCESS OF THAT AT WHICH SAID DISPERSION IS REMOVED FROM SAID BATH BY THE FABRICATING TECHNIQUE EMPLOYED, CAUSING THE UNSUED EXCESS OF SAID DISPERSION TO BE REMOVED FROM THE SURFACE OF SAID BATH OF ESSENTIALLY PLANAR FLOW, THE RATE AT WHICH THE INTRODUCED LATEX IS FED INTO SAID BATH BEING ESSENTIALLY CONSTANT AND ALSO BEING EQUAL TO THE TOTAL AMOUNT OF LATEX REMOVED THEREFROM. 